Manufacture of electrical rectifiers



Patented May 16, 1939 UNITED STATES PATENT OFFICE MANUFACTURE OFELECTRICAL RECTIFIE Philip H. Dowling, Forest Hills, Pa., assignor toThe Union Switch & Signal Company, Swissvale, Pa., a corporation ofPennsylvania No Drawing. Application December 27, 1937, Serial No.181,993

13 Claims.

neighborhood of about 1000 C. This heating of the blanks causes a layerof red or cuprous oxide to be formed on the blanks, and is continueduntil the cuprous oxide layer is of the desired thickness, a heatingperiod of from 8 to 13 minutes usually being sufficient to produce anoxide layer of the desired thickness. After a sufficient amount of oxidehas been formed on the blanks, the blanks are then transferredimmediately to a second furnace which is maintained at a temperature inthe neighborhood of 550 C., and are allowed to remain in this latterfurnace only for a suflicient length of time to permit them to cool downto the temperature of the second furnace. The oxidized blanks are thenremoved from the second furnace and are suddenly cooled or quenched asby plunging the blanks into cold Water or a current of cold air. Thisleaves the blanks with an inner coating of cuprous or red oxide ofcopper and a thin outer coating of black oxide of copper, the oxidecoating on the face which is not in contact with the contiguous face ofthe adjacent blank being much heavier than the oxide coating on theopposite face. Each blank is next treated to remove the black oxide fromboth faces of the blank, and the red oxide from the face which has thethinnest oxide coating. One process which is particularly suitable forremoving the excess oxide from the blanks is described and claimed inLetters Patent of the United States No. 2,094,642, granted to me onOctober 5, 1937. As a last step, the exposed face of the cuprous oxideremaining on the blanks is carbonized, as by rubbing into this facepowdered petroleum coke.

It is well-known that rectifiers manufactured by this process, as wellas by other well-known processes, exhibit the characteristic that when avoltage is applied to them in the high resistance direction, the high orblocking resistance, as it is generally termed, will decrease at firstrelatively rapidly and then somewhat more slowly with time until itattains an apparently stable value which may be considerably less thanthe initial value. The stable virtual value of the reverse current, whenan alternating voltage is applied to the rectifier, is increased due tothis effect, and this effect accordingly in some instances determinesthe voltage for which the rectifier is capable of effecting satisfactoryrectification in service. This change in blocking resistance with timeis ordinarily known as creep or reverse creep, and is usually temporaryin character, the apparent resistance in the reverse direction graduallyreturning toward its original value when the voltage is removed.

The decrease in resistance constituting the reverse creep appears to bedependent upon the physical and chemical features of the metal fromwhich the rectifier is manufactured. However,

, certain processes of manufacture will reduce this creep, and oneobject of my invention is to provide such a process.

According to one process which I have found to be useful for reducingreverse creep, the blanks after being cleaned are oxidized to an equaldegree on both sides by heating them in an oxidizing atmosphere at theusual oxidizing temperature long enough to form an appreciable coatingof cuprous oxide on the copper, a coating of one thousandth of an inchor more in thickness generally being sufficient. This preliminarycoating is then removed, preferably although not necessarily, byimmersing the blanks in a dilute solution of nitric acid. A 25% solutionby volume or less is satisfactory for this step in the process. Theblanks are immersed in the nitric acid solution for such alength of timethat the cuprous oxide is completely removed, but that the copper is notappreciably acted on by the acid. When the oxide has been properlyremoved in this manner, the blanks will be left with a highly polishedsurface which carries the microscopic traces of the outlines of thecuprous oxide crystals of the preliminary oxide coating. It ispreferable not to disturb this polished surface, but merely tothoroughly remove all traces of acid and finely divided copper broughtdown by ,the action of the acid. The water used to wash the blanksshould be reasonably free from impurities, it being preferable to usedistilled water for at least a final rinsing of the blanks. After theblanks have been thoroughly washed, the blanks are then oxidized asecond time by any process which would yield reasonably good rectifierswithout the use of the above described preliminary oxidation of theblanks, one process which is suitable for this second oxidation beingthe process described hereinbefore at the beginning of thespecification.

It should be pointed out that it is important that the preliminaryoxidation of the blanks should form a uniform oxide coating on bothsides of the blanks since otherwise, when the preliminary oxide coatingis removed preparatory to the second oxidation, the copper blanks willno longer be of uniform thickness and will not be fiat. Another methodwhich I have found to be satisfactory to reduce creep is to increase thelength of time the blanks are maintained in the oxidizing furnace in thehitherto generally employed process outlined at the beginning of thespecification. For example, rectifier blanks which are oxidized forapproximately 20 minutes, instead of the usual time of from 8 to 13minutes, at a temperature in the neighborhood of 1000 C. in an oxidizingatmosphere, prior to transferring them to the second or annealingfurnace, show a marked improvement in so far as creep is concerned, overblanks which are oxidized in the usual way. This last mentioned processis subject to the disadvantage that the increased oxidation timeincreases the thickness of the oxide coating, and thereby increases theresistance of the rectifier in the low resistance direction. However, incases where the stability of the resistance of the blanks in the highresistance direction is of more importance than the value of theresistance in the low resistance direction, this latter process is ofmaterial benefit.

Although I have herein shown and described only two processes ofmanufacture of electrical rectifiers embodying my invention, it isunderstood that various changes and modifications may be made thereinwithin the scope of the appended claims without departing from thespirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In the process preparing rectifier elements, the steps for reducingcreep which consist in forming on both sides of a copper blank :1 thincoating of cuprous oxide, and then removing this coating prior tosubjecting the blank to a subsequent oxidizing process.

2. The process of preparing a copper oxide rectifier element whichconsists in forming an oxide coating on both sides of a copper blank,removing this coating, and then forming a second oxide coating on theblank.

3. The process of preparing a copper oxide rectifier element whichconsists in forming a relatively thin oxide coating on both sides of acopper blank, removing this coating by the action of nitric acid, andthen forming a second oxide coating on the blank.

4. The process of preparing a copper oxide rectifier element whichconsists in forming a relatively thin oxide coating on both sides of acopper blank, immersing the blank in a dilute solution of nitric acidlong enough to completely remove this coating but not long enough toappreciably act on the copper, and then forming a. second oxide coatingon the blank.

5. The process at preparing a copper oxide rectifier element whichconsists in forming on both sides of a copper blank a cuprous oxidecoating having a thickness of approximately one thousandth of an inch,removing this coating, and then reoxidizing the blank. I

6. The process of preparing a copper oxide rectifier element whichconsists in forming a relatively thin cuprous oxide coating on bothsides of a copper blank, chemically treating the blank to remove thiscoating, and .then reoxidizing the blank. 1

7. The process 01' preparing a copper oxide rectifier element whichconsists in forming on both sides of a copper blank a coating of cuprousoxide, removing this coating by immersing the blank in a solution of 25%by volume of nitric acid, ,washing the blank to remove the acid and thefinely divided copper formed by the acid treatment, and then reoxidizingthe blank.

8. The process of preparing a copper oxide rectifler element whichconsists in forming on both sides or a copper blank a thin coating ofcuprous oxide, immersing the blank in a dilute solution of nitric acidjust long enough to remove this coating and leave the blank with ahighly polished surface, and then reoxidizing the blank.

9. The process 01' preparing a copper oxide rectifier element whichconsists in forming on both sides of a copper blank a thin coating ofcuprous oxide, immersing the blank in a dilute solution of nitric acidjust long enough to remove this coating and leave the blank with ahighly polished surface, washing the blank in water to remove all tracesof acid and finely divided copper formed by the acid treatment, and thenreoxidizing the blank.

10. The process of preparing a rectifier element which consists inheating a copper blank in an oxidizing atmosphere at a temperature 01'approximately 1000 C. to form a coating of cuprous oxide on both sidesof the blank, chemically treating the blank to remove all traces of theoxide coating, and then reheating the blank in an oxidizing atmosphereat a temperature oi approximately 1000 C. to form a coating of cuprousoxide on the blank.

11. The process of preparing a rectifier element which consists inheating a copper blank in an oxidizing atmosphere at a temperature ofapproximately 1000 C. to form a coating of cuprous oxide on both sidesof the blank, removing all traces of the oxide coating by subjecting theoxidized blank to the action of aweak solution of nitric acid, reheatingthe blank in an oxidizing atmosphere at a temperature of 1000 C. to forma coating of cuprous oxide on the blank, cooling the oxidized blank to atemperature of approximately 550 C., and then quenching said blank.

12. The process of preparing a rectifier element which consists inheating a copper blank in an oxidizing atmosphere at a temperature ofapproximately 1000 C. for a predetermined period of time to form acoating of cuprous oxide on both sides of the blank, chemically treatingthe blank to remove the oxide, reheating the blank in an oxidizingatmosphere at a temperature oiapproximately 1000 C. for a predeterminedperiod of time to form a coating of cuprous oxide on the blank, coolingthe blank to a temperature of approximately 550 C., and then quenchingsaid blank to cool it.

13. The process of preparing a rectifier element which consists inheating a copper blank in an oxidizing atmosphere at a temperature ofapproximately 1000 C. to form a coating of cuprous oxide on both sidesof the blank, removing all traces oi the oxide coating by subiecting theoxidized blank to the action of a weak solution of nitric acid, washingthe blank to remove all traces of the nitric acid and any finely dividedcopper deposited on the blank by the action of the acid, reheating theblank in an oxidizing atmosphere at a temperature of 1000 C. to form acoating of cuprous oxide on the blank, cooling the oxidized blank to atempera ture of approximately 550 C., and then quenching said blank.

PHILIP H. DOWLING.

